The present invention relates to graphite nanofibers, a source of electron emission and a method of the preparation thereof, a display element equipped with such an electron-emitting source as well as a lithium ion secondary battery. More specifically, the present invention pertains to graphite nanofibers, which may be applied to an electron-emitting source used in the field of display devices; an electron-emitting source provided with such graphite nanofibers; a method for preparing such an electron-emitting source according to the thermal chemical vapor deposition (thermal CVD) technique; a display element equipped with such an electron-emitting source; a carbonaceous material for negative electrodes of batteries, which consists of the graphite nanofibers; and a lithium ion secondary battery, which makes use of the carbonaceous material for negative electrodes as an active material for the negative electrode. The electron-emitting source can ensure a high quantity of emitted electrons and can be used not only in flat panels such as FED""s, but also as an electron source for the conventional CRT""s.
FIG. 1 shows the construction of a typical cold cathode ray source. The term xe2x80x9ccold cathode ray sourcexe2x80x9d means a cathode (or a negative electrode) serving as an electron-emitting source, which can emit electrons without application of any heat. In this case, a conical cathode chip (such as those comprising W, Mo, Si or the like) can be formed on an electrode substrate, for instance, by first applying a metal electrode substrate 2 (comprising, for instance, W, Mo or Si) onto a substrate 1, then forming a dielectric film serving as an electrical insulator 3 and a metal gate film (comprising, for instance, W, Mo and/or Si) serving as a gate electrode 4, on the electrode substrate 2, forming a resist film thereon, forming a hole pattern according to, for instance, a photolithography technique, and then removing the metal gate film and the dielectric film immediately below the holes through etching to thus expose the electrode substrate 2. Then a substance is obliquely deposited on the substrate while rotating the electrode substrate around a line, serving as a central line, vertical to the substrate to thus give a conical negative electrode chip 5. If Mo as an emitter material is, for instance, deposited on the electrode substrate, the direction of the Mo deposition is controlled in such a manner that Mo atoms may be deposited within the holes, while the Mo atoms gradually fill up the holes and thereafter, a release film is removed together with the excessive Mo film deposited on the substrate other than the holes to thus give an emitter. If the emitter prepared according to this method is used in the field of display, however, it can, at present, be operated only at an electric field on the order of 100 V/xcexcm.
As has been discussed above, there have conventionally been investigated, for instance, Si and/or Mo as materials for cathodes (or negative electrodes), but there has recently been investigated the use of carbon nanotubes as such a cathode material. A carbon nanotube is a graphite fiber having a cylindrical shape formed from a helical structure mainly composed of carbon 6-membered rings and having a multiple structure in which a plurality of quite fine cylinders are concentrically arranged and either of the ends thereof is opened. The nanotube having such a structure is excellent in various characteristic properties such as electron emission characteristic properties, heat resistance and chemical stability as compared with those observed for other metallic materials. Such a nanotube has in general been produced according to a variety of methods such as an arc discharge technique, a laser evaporation technique and a plasma CVD technique. Among these., a method for preparing carbon nanotubes, which makes use of a microwave CVD technique, permits the growth of a carbon nanotube on a specific substrate perpendicularly to the substrate. The cold cathode ray source (electron-emitting source) has only a low quantity of electrons emitted at an applied voltage of 3 V/xcexcm on the order of 1 mA/cm2.
In addition, there has recently been required for the development of a battery having a high energy density in proportion to the miniaturization of electronic devices. For this reason, a high quality lithium ion secondary battery has been developed. For instance, there has been proposed a lithium ion secondary battery excellent in the cycle life and having a high discharge (service) capacity, which can be produced through the use of a carbonaceous material for the negative electrode such as the foregoing carbon nanotubes each having monolayered wall surface. This method makes the best use of the fact that an intercalation carbon compound of lithium can easily be formed electrochemically. More specifically, if charging a lithium ion secondary battery provided with a carbon negative electrode in a non-aqueous electrolyte, the lithium in the lithium-containing positive electrode is electrochemically doped between the carbon layers of the negative electrode, the lithium-doped carbon layer thus serves as a lithium electrode, the lithium is de-doped from the carbon layers as the discharge of the battery proceeds and as a result, it returns back to the positive electrode. In this respect, the charging rate (mAh/g) of the carbonaceous material per unit weight is determined by the amount of doped lithium and therefore, it is necessary to increase the degree of lithium-doping of the negative electrode as high as possible in order to ensure a high charging rate of the battery.
In the case of the electron-emitting sources comprising the conventional carbon nanotubes including those obtained by growing carbon nanotubes on a substrate perpendicularly thereto according to the foregoing methods, electrons are emitted from the carbon nanotubes through the tip or defective portions thereof. For this reason, any conventional carbon nanotubes cannot presently be used in the applications such as an electron source for CRT""s, which require a high current density.
Moreover, in the case of the lithium ion secondary battery, which makes use of the foregoing carbon nanotubes as a negative electrode material, the carbon nanotube should have an open end or defective portions in order to ensure the intercalation of lithium ions into the nanotube. However, the conventional carbon nanotubes are not always sufficient in this respect. In other words, the conventional carbon nanotubes cannot permit the intercalation of lithium ions in a desired sufficient quantity or the amount of doped lithium cannot be increased to a desired level and therefore, the resulting electron-emitting source does not have any satisfactorily long cycle life and fast charging ability.
Accordingly, an object of the present invention is to solve the foregoing problems associated with the conventional techniques and more specifically to provide a negative electrode material ensuring a high electron emission density and an ability of emitting electrons at a low electric field, which have never or less been attained by the carbon nanotube; a carbon-based electron-emitting source comprising the negative electrode material and a method of the preparation thereof; a display element equipped with such an electron-emitting source; a negative electrode carbonaceous material for batteries having a high quantity of doped lithium and a lithium ion secondary battery, which makes use of the carbonaceous material as an active material for the negative electrode thereof and which thus has a sufficiently long cycle life, a fast charging ability and a high service capacity.
The inventors of this invention have conducted various studies to develop a negative electrode material, ensuring a high electron emission density and an ability of emitting electrons at a low electric field, and a carbonaceous material usable as an active material for the negative electrode of a lithium ion secondary battery, which makes use of the carbonaceous material as an active material for the negative electrode thereof and which thus has a sufficiently long cycle life, a fast charging ability and a high service capacity. The inventors have found that a graphite nanofiber whose structure has never been reported can be obtained during the growth of a crystal starting from a carbon-containing gas and hydrogen gas using the thermal CVD technique and that the graphite nanofiber possesses excellent electron emission characteristics and excellent quality as an active material for negative electrodes used in the lithium ion secondary battery and have thus completed the present invention on the basis of the foregoing findings.
According to an aspect of the present invention, there is provided a graphite nanofiber having a cylindrical structure in which graphene sheets each having an ice-cream cone-like shape whose tip is cut off are put in layers through catalytic metal pieces or particles; or a structure in which small pieces of graphene sheets having a shape adapted for the surface shape of a catalytic metal piece or particle are stacked on top of each other in layers through the catalytic metal particles. Among these, the graphite nanofiber having a cylindrical structure preferably has a through hole, which is vacant or filled with amorphous carbon, and has the diameter thereof preferably ranging from 10 nm to 600 nm. The graphite nanofiber having a diameter of less than 10 nm has not yet been prepared. On the other hand, those having a diameter of more than 600 nm are insufficient in the electron emission characteristics. It is preferred that the foregoing catalytic metal comprises Fe, Co or an alloy including at least one of these metals. The foregoing graphite nanofiber is effective as a negative electrode material having excellent electron emission characteristics such as a high electron emission density and an ability of emitting electrons at a low electric field.
According to a second aspect of the present invention, there is provided an electron-emitting source, which comprises a carbon layer formed on the surface of an electrode substrate or on the patterned surface portions of a patterned electrode substrate, wherein the carbon layer comprises the graphite nanofiber having the foregoing structure. In this respect, it is preferred that the electrode substrates on which the foregoing carbon layer is formed are those comprising Fe, Co or an alloy including at least one of these metals. These metals have a catalytic effect for forming a graphite nanofiber. An electron-emitting source provided with the graphite nanofiber would show excellent electron emission characteristics such as a high electron emission density and an ability of emitting electrons at a low electric field.
According to a third aspect of the present invention, there is provided a method for preparing an electron-emitting source, which comprises the step of growing a carbon layer on the surface of an electrode substrate or on the patterned surface portions of a patterned electrode substrate, at a layer-forming temperature, which does not exceed the heat resistant temperature of the electrode substrate comprising Fe, Co or an alloy including at least one of these metals, using a carbon-containing gas and hydrogen gas according to the thermal CVD technique to thus give a growth layer of graphite nanofibers having the foregoing structure. Those having such growth layer of the graphite nanofibers may serve as electron-emitting sources and may constitute cold negative electrode sources.
The electron-emitting source according to the present invention may likewise be prepared by collecting graphite nanofiber powders or particles having the foregoing structure, dispersing them in a solvent to form a paste and then applying the resulting paste onto an electrode substrate; or immersing an electrode substrate in a dispersion obtained by dispersing the foregoing powders or particles in a solution and then depositing or adhering the powders or particles, onto the substrate through the electro-depositing technique to give a desired electron-emitting source.
According to a fourth aspect of the present invention, there is provided a display element, which comprises a plurality of transparent conductive films having a desired pattern, an electron-emitting source formed by applying a carbon layer comprising graphite nanofibers having the foregoing structure on the patterned surface portions of a patterned electrode substrate, and a luminous body opposed to the carbon layer. As mentioned above, this display element is so designed that the carbon layer and the luminous body are arranged in such a manner that they are opposed to one another. For this reason, if arbitrarily selecting the carbon layer and the transparent conductive film and an electric voltage is applied thereto, electrons are emitted from the carbon layer and only a specific portion of the luminous body thus emits light.
According to a fifth aspect of the present invention, there is provided a negative electrode carbonaceous material for batteries, which consists of graphite nanofibers having the foregoing structure and which is doped with lithium in a high rate. The graphite nanofiber having a diameter of more than 600 nm never shows desired quality such as a high capacity. Such a graphite nanofiber has a fine structure approximately identical to that of the conventional carbon nanotube and accordingly, not only has characteristic properties such as those observed for active carbon having a high specific surface area, but also has a plurality of opened faces, which allows free passage of lithium ions therethrough. Therefore, the graphite nanofiber may serve as an excellent active material for negative electrodes having high charging and discharging capacities of more than the theoretical capacity of graphite (372 mAh/g).
According to a sixth aspect of the present invention, there is provided a lithium ion secondary battery, which comprises a positive electrode including, as an active material for positive electrode, a lithium transition metal oxide; a negative electrode including a carbonaceous material as a negative electrode active material; and an organic solvent-based electrolyte, wherein the carbonaceous material essentially consists of the graphite nanofibers having the foregoing structure. The use of such a carbonaceous material would permit the production of a lithium ion secondary battery having a long cycle life, a fast charging ability and a high service or discharge capacity.